Conveyor belts are in common use in mines, for the transport of ore to and from crushing, milling, and comminuting machines. Dust and fine particles of crushed ore can become caked very firmly indeed to the belt, especially if moisture is present.
Scraping caked material from mine conveyor belts is well-known to be a most demanding task. The task is usually approached on the basis of pressing a scraper blade against the (moving) face of the belt. The scraper blade gradually wears down under the abrasive conditions, and one of the problems of belt-scraping lies in providing a scraper blade of sufficient length that the blade has a long service life between replacements.
The conventional approach to the problem that the blade has only a short life is to press the blade against the belt with less force. The invention is aimed at providing an economical scraper in which the blade has a long service life, yet in which the blade is pressed against the belt with sufficient force for excellent scraping.
U.S. patent publication no U.S. Pat. No. 5,048,667 (MORIN, 17-Sep.-91) shows a scraper assembly in which the scraper blade is of flexible plastic material, which is rolled into a coil. This arrangement has led to a scraper assembly in which the scraper blade need only be changed on, say, an annual basis, as against the every-six-weeks replacement which has tended to be the industry standard in the most demanding situations.
One of the problems that has arisen in regard to these coiled-blade scraper assemblies is that the blade can tend to stick or "freeze" to the inside of the blade housing. As is shown in '667, the coiled blade is contained inside a cylindrical housing. The inherent stiffness of the blade material means that the blade tends to try to unroll itself within the housing; in fact, the blade tends to uncoil itself until the material of the blade lies in direct touching contact with the inside of the housing.
Once the blade has unrolled itself as much as it can inside the housing, the friction of the blade material against the inside of the housing builds up, and this friction can make it difficult for the blade to move and rotate within the housing.
If the blade is allowed to unroll inside the housing, ie if the blade is allowed to unroll itself and make frictional contact against the inside wall of the housing, the force required to drive the blade out of the housing therefore increases; not only is the need for the increased force a disadvantage, but another disadvantage is that some of the controllability of the pressure of the blade against the belt is lost.
It may be noted that the friction is caused by the tendency of the belt to uncoil itself, rather than by the pressure forcing the blade against the belt.
In the said patent '667, the blade is driven out of the housing and against the belt by means of fluid pressure inside the housing. One benefit of this system is that the reaction to the belt contact force is not supported by direct touching contact between the coil and the housing, which eliminates a source of what would be a considerable friction. However, the material of a scraper blade, even though able to be wound into a coil, nevertheless is characterized as being very stiff and intractable; where the material does touch the housing as a result of its trying to uncoil itself, the friction arising from that contact can be considerable.
Thus, in '667, the blade is forced out of the housing in such a manner that the actuation force on the blade does not give rise to a frictional resistance proportional thereto: the friction that arises simply due to the tendency of the blade to uncoil itself inside the housing, is the problem with which the invention is concerned. The problem exists because the coil of blade material tends to unwind itself until it comes into contact with the housing, or with whatever is present that prevents it unwinding further.
The present invention is aimed at providing a scraper assembly of the coiled-blade type, in which friction associated with contact between the coiled blade and the blade housing is much less than has been the case in the previous scraper assemblies.
Another aim of the invention is to provide a scraper assembly in which replacement of the coiled blade is made more simple, by the fact that the replacement blade can be assembled in the factory into a coiled-up state, and can remain so during insertion of the new blade into the housing, whereby the service technician does not have to coil up the blade during or after assembly of the blade into the housing.
It is an aim of the invention to keep the blade material coiled tightly enough that the coil cannot "escape" and start to touch the inside of the housing. On the other hand, it is an aim of the invention that the force required to draw the free end of the blade out of the coil should be a light force, and that the force should be as light when the coil is new as when the coil is almost worn away.
It is an aim of the invention to keep the coil of blade material constrained against expansion without resorting to applying torque to the coil. It would be theoretically possible, for example, to keep a coil tight by applying a torque to a spindle to which the inner end of the spiral coil is attached. However, it is preferred that the coil be kept under constraint by means which do not require a torque to be exerted between the coil and the housing: such a torque requirement, if present, would greatly interfere with the freedom of choice of the designer to design an adequate actuator means for pressing the blade against the belt.
It will be understood that if the constraint against unrolling were to take the form of a cage of some kind, placed so as to encircle the coil, the coil would simply unroll itself as much as possible within the cage. The friction of the partially unrolled coil against the cage then would be hardly any less damaging than the friction of the partially unrolled coil against the housing. Therefore, the means for constraining the coil against unrolling cannot, in practice, take the form of a coil-encircling cage.
It might be considered that it would be possible to position suitable means on the walls of the housing to hold the coil away from the walls, and to thereby stop the coil expanding. However, to avoid installation difficulties, it is best for the coil containment constraints to be provided, not by or from the housing, but in the coil assembly itself, ie in the assembly that is installed with the blade.